Recently, since a wireless communication system uses a plurality of different mixing frequencies, services using adjacent frequencies frequently cause crosstalk.
In order to prevent the crosstalk, when a wireless communication system transmits via one service frequency, it uses a band stop filter stopping other service frequencies to reduce crosstalk between frequencies.
According to the related art, there is a short parallel stub band stop filter formed from microstrip.
However, since the filter has a periodic frequency response characteristic and phase response characteristic, there is a problem of unnecessary resonance at a portion where a phase is shifted by 180°. Further, since the filter uses a stub of a λ/4 length, the size of the stub increases as the resonance frequency gets lower.
For this reason, in order to complement these disadvantages, the filter having various structures has been suggested. In particular, researches on a transmission line having a composite right/left handed (CRLH) structure, in which a left handed material (LHM) and a right handed material (RHM) are mixed are carried on. However, the CRLH structure is limited only to a specific frequency so far.
Further, with the development of portable devices for the wireless communication system, a demand for miniaturization of a device having a band stop filter is increasing, which is hard to achieve by the related art filter.